Design and modulation of active sites for oxygen-related energy conversion reactions : case studies of Ni-, Co- and Fe-based electrocatalysts

Abstract

Growing need for sustainable energy sources has driven the extensive exploration of energy conversion technologies for several decades, especially electrochemical water splitting and metal-air batteries. Several critical electrocatalysis involved in these applications have attracted a great attention, in particular, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). To overcome the thermodynamic barriers and accelerate the kinetics of electrode reactions, the potential electrocatalysts with required properties such as high efficiency, durability, and low cost are highly desired. Compared with traditional precious metal-based materials (e.g., Pt and Pd for HER/ORR; Ir and Ru for OER), a group of transition metal-based (Mn, Fe, Co, Ni, Cu, etc.) nanomaterials were found to exhibit competitive activities and excellent stabilities. However, the delicate design and fabrication of functional catalysts remain challenging to date, and the related catalytic mechanisms are still not clearly understood. The catalytic performances of transition metal-based nanomaterials family such as oxide, (oxy)hydroxide, nitride, and chalcogenide are determined by the intrinsic electronic properties as well as the microstructure and surface conditions. The specific mechanism and rate-limiting step for each catalysis vary largely, making the understanding and design of multi-functional electrocatalysts more difficult. This thesis reports several electrocatalysts developed for these important energy conversion reactions (HER/OER/ORR). Combining the well-designed experiments, ex situ/in situ characterization techniques, and theoretical calculations, their structural transition, catalytic activities, and reaction mechanism were investigated in detail. The research findings are listed as follows: 1. 2D CoNi bimetallic organic frameworks (CoNi-MOF) were directly grown as nanoplate array on a copper foil under hydrothermal conditions. The 2D CoNi-MOF demonstrated highly active OER catalysis with a small overpotential and Tafel slope in alkaline media. The superior catalytic activity was shown to be correlated to the dominant (220) facets, which caused the lattice contraction and enhanced conductivity along the Z-axis, and hence facilitated the electron transfer from substrate to active catalytic centers. Moreover, annealing the CoNi-MOF in NH3 atmosphere yielded a hybrid nanoplate array of metallic nitrides on amorphous carbon network (CoNiN@C), which efficiently and stably catalyzed HER. The overall water splitting using these 2D CoNi-MOF-based catalysts showed a high Faradaic efficiency. 2. Despite recent attempts using metal-organic frameworks (MOFs) directly as electrocatalysts, the electrochemical stability of some MOFs and the role of in situ formed species are elusive. The comprehensive discussion on the evolution of zeolitic imidazolate framework-67 (ZIF-67) during OER are presented. Dramatic morphological changes exposed electron-accessible Co sites during the electrochemical treatments. The extensive conversion of tetrahedral Co sites in ZIF-67 to tetrahedral α-Co(OH)2 and octahedral β-Co(OH)2, as well as the formation of their corresponding oxidized forms were observed. Subsequent OER analyses suggested the CoOOH species converted from α/β-Co(OH)2 as the dominating active sites with the CoOOH from α-Co(OH)2 being more active. 3. Transition metal (oxy)hydroxide are the active phases of many common OER electrocatalysts. The studies of double layered hydroxides (LDH) can provide an insightful understanding of "structure-properties" relationship of electrocatalyst. Interface and doping engineering, two efficient modification strategies, were engaged to prepare hybrid Ce(OH)3@NiFe-LDH and Ce-doped NiFe-LDH nanosheet array composites in attempt to understand the roles of foreign elements in catalyzing OER. The electronic structures were shown to be modulated at different level by either the Ce dopant or forming interface. Operando Raman studies demonstrated the introduction of Ce and Fe could stimulate the oxidation of Ni3+ to Ni4+. Comparing with the pristine NiFe-LDH and Ni(OH)2, two modified catalysts exhibited and better OER performances with adjusted binding energies for intermediates. 4. Single atomic metal-N-C materials have attracted immense interest as a promising electrocatalyst for ORR. However, the attention has been focused only on the coordination of metal atoms, overlooking the possibility of cooperative catalysis by multiple active sites. Fe single atoms and clusters co-embedded in N-doped carbon (Fe/NC) were successfully prepared. The synergistic effect of Fe single atoms and clusters resulted in the performance enhancement in pH-universal ORR catalysis via the four-electron pathway. Combining a series of experimental and computational analyses, the geometric and electronic structures of catalytic sites in Fe/NC were revealed and the neighboring Fe clusters were shown to weaken the binding energies of ORR intermediates on Fe-N sites, hence enhancing both catalytic kinetics and thermodynamics. The Fe single atom/cluster catalytic system exhibited excellent ORR catalytic performance in both alkaline and acidic media.